The basic characteristics of electronically commutated reluctance motors operated as continuously rotating variable-speed drives are well known, since they are members of the class of variable reluctance motors, commonly used in stepper motor applications. As variable-speed drives, electronically commutated reluctance motors are designed for efficient power conversion rather than for particular torque or control characteristics typically required in stepper motor applications, and the pole geometry and control strategies differ accordingly. For example, the number of rotor teeth is relatively small in an electronically commutated reluctance motor (cf., variable reluctance stepper motors), giving a large step angle, and the conduction angle is, generally, modulated as a function of both speed and torque to optimize operation as a variable-speed drive. As a term of art, these variable-speed reluctance motors are generally known as switched reluctance motors.
Because of recent developments in power semiconductor devices such as power MOSFETs and insulated gate thyristors (IGTs), switched reluctance motors have gained attention relative to other types of motors suitable for variable-speed drive applications. This increased attention derives from the fact that switched reluctance motors compare very favorably with other types of motors typically used as variable-speed drives. For example, the speed versus average torque curves for switched reluctance motors are very similar to the curves for brushless permanent magnet (PM) motors--e.g., the curves are fairly linear with no discontinuities of torque. Additionally, switched reluctance motors are the cheapest type of motor to manufacture. They are rugged and robust and therefore well suited for heavy duty use. They have excellent heat dissipation qualities, and they do not require brushes or slip rings. The drive circuits for switched reluctance motors are the simplest and lowest cost compared to drives for other motors. Moreover, using state-of-the-art semiconductor technology for controllers, the efficiency of switched reluctance motors compares very favorably with other classes of variable-speed motors such as inverter-driven AC motor and PM motors.
Although the foregoing comparative features are favorable, switched reluctance motors are also known to have several disadvantages which are common to all variable-speed drive motors. Specifically, copper, hysteresis and eddy current losses limit motor efficiency, especially at relatively high RPMs.
A recent advance in the design of switched reluctance motors described in U.S. patent application No. 232,436 has significantly improved performance characteristics of such motors, especially at higher RPMs (e.g., 10,000 and up). Unique structural and excitation features of this motor significantly reduce hysteresis and eddy current losses relative to conventional switched reluctance motors. Electronically commutated reluctance motors as illustrated in the '436 application are hereinafter referred to as ECR motors. These ECR motors are typically characterized by a stator and rotor mounted for relative rotation wherein the rotor has evenly spaced teeth and the stator has unevenly spaced poles such that when the poles of the stator are energized by a single phase of a power source, they define pairs of adjacent poles, with the poles of each pair having opposite polarities. In the embodiments of the ECR motor illustrated in the '436 application, the stator is a stack of laminations, where each lamination is a plurality of poles supported on a yoke as is conventional in the art of motor design. Windings are wrapped about the poles of the stator in a manner which allows each phase of a power source to energize pairs of adjacent poles having opposite polarities so as to create a magnetic circuit between each of the pole pairs. Both poles of a pair are always excited together in any energization scheme utilized to drive the ECR motor, thereby ensuring the primary magnetic circuit formed by the pair is through the back iron area of the stator yoke bridging the poles of the pair.
When more than one pair of adjacent stator teeth are energized at a time using a unipolar drive, "secondary magnetic circuits" are created linking two pairs of stator teeth by way of a flux path that crosses the "primary magnetic circuit" of unenergized pairs of stator teeth (a primary magnetic circuit is the magnetic circuit between poles of a pair). These secondary magnetic circuits are formed in part by the continuous back iron or yoke of the stator, and they effectively increase the flux switching frequency for those portions of the stator back iron where primary and secondary circuits overlap. Depending upon the mode of excitation and the particular configuration of the ECR motor, these secondary circuits may have little or great effect on the performance characteristics of the ECR motor.
To prevent the occurrence of such secondary magnetic circuits linking pairs of stator teeth, a bipolar drive may be substituted for the unipolar drive in order to control the relative polarities of simultaneously polarized pairs in a relationship such that neighboring poles separated by an unenergized pair or pairs of poles are of the same polarity. The use of bipolar drives with an ECR motor, however, increases the system cost relative to conventional switch reluctance motors using unipolar drives. The increased efficiency provided by an ECR motor is significant relative to a switch reluctance motor and, on an objective basis, the use of a bipolar drive should not deter a designer from choosing an ECR motor for a variable-speed application. Nevertheless, it would be desirable to provide a design for an ECR motor that realizes the full efficiency available from such a motor without the need for a bipolar drive when the motor is driven by certain energization schemes.
A related cost comparison between ECR motors and conventional switched reluctance motors is the cost of manufacturing. Assuming the use of similar manufacturing techniques, ECR and conventional switched reluctance motors cost approximately the same to produce. One of the major expenses of manufacturing either type of motor is the cost of preparing windings for the stator lamination stack. The windings must be placed over the stator poles which extend inwardly from the generally cylindrical surface formed by the stack. The relatively small inner diameter of the stator makes the production of windings a difficult task which requires expensive machinery for high volume production or expensive hand labor if volume demand is less. To complement the superior performance characteristics of an ECR motor, it would be desirable to provide for construction of an ECR motor which is inherently less expensive to manufacture than conventional switched reluctance motors.